As for image sensors, such as a chip size package (CSP), etc., a mode of protection by sticking a glass substrate onto a silicon substrate is known, and a glass for silicon pedestal in which an elongation percentage by thermal expansion is made close to an elongation percentage by thermal expansion of a silicon substrate to be bonded to a glass is proposed (see, for example, Patent Document 1).
In addition, in the semiconductor assembly process so far, a series of assembly processes in which after cutting a silicon substrate and a glass substrate in a wafer state, respectively, the silicon substrate and the glass substrate are stuck to each other, followed by die bonding, wire bonding, molding, and so on, are performed. In recent years, the wafer-level package technology in which a silicon substrate and a glass substrate in a full-scale wafer state are stuck to each other and subjected to an assembly process, followed by cutting is in the limelight as a next-generation CSP technology.
In order to stick the silicon substrate and the glass substrate to each other, a heat treatment step is required. In the heat treatment step, for example, a laminated substrate obtained by sticking the silicon substrate and the glass substrate to each other at a temperature of 200° C. to 400° C. is subjected to temperature decrease to room temperature. At this time, if there is a difference in thermal expansion coefficient between the silicone substrate and the glass substrate, a large residual strain (residual deformation) is caused in the silicon substrate due to the difference in thermal expansion coefficient.
In the wafer-level package technology, since the silicon substrate and the glass substrate are stuck to each other in the wafer state, even as for the difference in thermal expansion coefficient, which has not been problematic so far, the residual strain is liable to be caused in the silicon substrate. In addition, in the heat treatment step, it is required that an alkali ion is not diffused from the glass substrate into the silicon substrate.